The present disclosure relates to systems and methods for drilling and/or treating subterranean formations that include shale.
Treatment fluids can be used in a variety of subterranean treatment operations. As used herein, the terms “treat,” “treatment,” “treating,” and grammatical equivalents thereof refer to any subterranean operation that uses a fluid in conjunction with achieving a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid. Illustrative treatment operations can include, for example, fracturing operations, gravel packing operations, acidizing operations, scale dissolution and removal, consolidation operations, and the like. For example, a fluid may be used to drill a well bore in a subterranean formation or to complete a well bore in a subterranean formation, as well as numerous other purposes. A drilling fluid, or “mud” which a drilling fluid is also often called, is a treatment fluid that is circulated in a well bore as the well bore is being drilled to facilitate the drilling operation. The various functions of a drilling fluid include removing drill cuttings from the well bore, cooling and lubricating the drill bit, aiding in support of the drill pipe and drill bit, and providing a hydrostatic head to maintain the integrity of the well bore walls and prevent well blowouts.
During drilling of subterranean well bores, various strata that include reactive shales may be encountered. As used herein, the term “shale” is defined to mean materials that may “swell,” or increase in volume, when exposed to water. Examples of these shales include certain types of clays (for example, bentonite). Reactive shales may be problematic during drilling operations because of, inter alia, their tendency to degrade when exposed to aqueous media such as aqueous-based drilling fluids. This degradation, of which swelling is one example, can result in undesirable drilling conditions and undesirable interference with the drilling fluid. For instance, the degradation of the shale may interfere with attempts to maintain the integrity of drilled cuttings traveling up the well bore until such time as the cuttings can be removed by solids control equipment located at the surface.
Shale disintegration also may impact “equivalent circulating density” (“ECD”). ECD may be affected by the solids content of the drilling fluid, which may increase if surface solids control equipment cannot remove shale from the drilling fluid. Plastic viscosity (an indicator of size and quantity of solids) is an important parameter that affects drilling rate. Maintenance of appropriate ECD is important in drilling a well bore where a narrow tolerance exists between the weight of the drilling fluid needed to control the formation pressure and the weight of the drilling fluid that will fracture the formation. In such circumstances, minimizing shale degradation may be desirable, inter alia, to control of the viscosity of the drilling fluid. Moreover, degradation of drilled cuttings prior to their removal at the surface may prolong drilling time because shale particles traveling up the well bore can break up into smaller and smaller particles, which can expose new surface area of the shale particles to the drilling fluid and lead to further absorption of water and degradation.
Shale degradation may substantially decrease the stability of the well bore, which may cause irregularities in the diameter of the well bore, e.g., the diameter of some portions of the well bore may be either smaller or greater than desired. In an extreme case, shale degradation may decrease the stability of the well bore to such an extent that the well bore collapses. Degradation of the shale also may interrupt circulation of the drilling fluid, cause greater friction between the drill string and the well bore, and/or cause the drill string to become stuck in the well bore. Accordingly, the complications associated with shale swelling during drilling may substantially increase the time and cost of drilling.
One technique used to counteract the propensity of aqueous drilling fluids to interact with reactive shales in a formation involves the use of certain additives in aqueous drilling fluids that may inhibit shale, e.g., additives that may demonstrate a propensity for reducing the tendency of shale to absorb water. Amphoteric materials (i.e., substances that may exhibit both acidic and/or alkaline properties) are one type of water-based shale inhibitor that has been used in the past. Amphoteric materials are believed to attach to the shale substrate, thus preventing water ingress. However, amphoteric inhibitors may be environmentally undesirable, especially in heavily regulated areas, because they typically demonstrate low biodegradability and high toxicity. Potassium chloride is another conventional shale-inhibiting component. However, potassium chloride may only be moderately effective at inhibiting shale swelling in some cases, and can be environmentally unacceptable in certain areas of the world since high concentrations of potassium ions may harm certain types of marine life or contaminate aquifers. Polyglycols have also been used as shale inhibitors in water-based drilling fluids, but have not demonstrated satisfactory inhibition levels. Partially hydrolyzed polyacrylamides (PHPA) and polyvinylpyrrolidone (PVP) have also been utilized in many regions, but these have been found to have undesirable properties in certain circumstances.
While embodiments of this disclosure have been depicted, such embodiments do not imply a limitation on the disclosure, and no such limitation should be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and not exhaustive of the scope of the disclosure.